1. Field of the Invention
The present invention relates generally to plasma magnetrons that are utilized in sputtering processes, and more particularly to magnetrons having a hollow cathode wherein the object to be coated passes through the center of the hollow cathode.
2. Brief Description of the Prior Art
Vacuum metallization by sputtering is commonly accomplished by a planar magnetron configuration because of the high rates this technique offers compared to other sputtering techniques. In the planar magnetron, a planar target is backed with magnets to enhance and confine the plasma. A limitation of planar magnetrons is that the sputtering is most aggressive near the plasma zones, and thus the target is inefficiently utilized. Secondly, the power and thus the deposition rate is limited by the maximum temperature which the target sustains. Therefore, for high throughput systems an incentive exists for increasing cooling effectiveness. Another limitation of planar magnetrons is that they are relatively wasteful when attempting to deposit films upon fibers or other small objects.
Recently, improvements to such systems have been built which utilize a rotating cylindrical cathode target with fixed interior magnets. Such a device is taught in U.S. Pat. No. 4,356,073 issued Oct. 26, 1982 to McKelvey. The result is a uniform etch of the outer surface of the cylindrical cathode target across most of its length, offering efficient use of the target. Secondly, run times between target changes are increased dramatically. If target cooling is accomplished by passing water through the center of the cylindrical target the power can be increased, potentially raising the sputtering rate due to more efficient cooling. However, as with planar targets, significant waste occurs when attempting to deposit thin films upon fibers and other small objects.
In order to improve the coating of the fibers, yarns, wires and other continuous objects a hollow cathode target configuration has been employed successfully. The hollow cathode has a cylindrical target which is designed to sputter in the interior. The advantages of the hollow cathode is that most of the over spray collects on the opposite side of the target, dramatically increasing the material usage. Hollow cathodes are also advantageous because cooling the outside of the cylinder is very efficient, and because the system sputters all around the substrate, resulting in a uniform coating.
Magnetically enhanced hollow cathode magnetrons exist in which cylindrical electromagnets are placed concentrically along portions of the length of the target. Such a device is taught in U.S. Pat. No. 4,966,677, issued Oct. 30, 1990 to Alchert et al. The target is consumed at the greatest rates between the magnets where the plasma is most intense. Sectioned cathodes have been used which minimizes affect on material usage by swapping highly worn sections with those which have little wear. A drawback of such devices is that targets are costly due to additional machining, and the system must be shut down to be rebuilt during or between runs.